1. Field of the Invention
The present invention relates to a digital automatic gain control (to be simply referred to as AGC hereinafter) linearizer for performing digital automatic gain control with high linearity in mobile communications and, more particularly, to a digital AGC linearizer having a predistortion function even with a low-linearity, low-cost AGC element.
2. Description of the Prior Art
Modulation/demodulation techniques used in mobile communication systems include FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access). Any system performs power control for an uplink signal to suppress energy radiation inside or outside a cell or outside an assigned channel to an allowable value or less, thereby maximizing the system capacity. However, particularly in the CDMA system, power control greatly influences the system capacity, so power control must be done carefully in order to maximize the system performance.
In general, a mobile communication terminal which receives a digitally modulated signal has a variable gain element having a gain adjusted by a control signal. Adjusting the gain in accordance with the level of the received signal is called AGC. In digital mobile communications, AGC (Auto Gain Control) is done by detecting the power of an output from the variable gain element.
The detected value is generally compared with a reference value as desired power to generate an error signal. The error signal is used to control the variable gain element to obtain the desired power.
To optimize the SN ratio in digital demodulation, the linearity of an AGC amplifier must be kept substantially over the dynamic range of an analog-to-digital converter (to be referred to as ADC) in the baseband. This means that the AGC amplifier must be able to operate over the whole dynamic range.
On the other hand, the transmit AGC in CDMA includes open-loop power control and closed-loop power control based on a command from the base station. A transmit AGC amplifier is connected to the power amplifier input of a mobile terminal, whose power amplifier output is connected to an antenna duplexer.
In the reception section, a signal received by the antenna is input to a low noise amplifier (to be referred to as LNA) via the antenna duplexer, and the signal amplified by the LNA is input to a receive AGC element. In this way, the AGC element is controlled by the power of the received signal, and an output from the AGC element is sent to the baseband and demodulated into a digital signal.
In the CDMA mobile terminal, the transmission power is determined on the basis of estimation at only the terminal in open-loop control, and on the basis of a command from the connected base station in closed-loop control. In open-loop estimation at only the terminal, the strength of the signal received from the base station is detected, uplink and downlink radio channels are regarded to be in the same state, and both the receive and transmit AGCs are kept at desired operation points by the same controlled variable.
In an actual mobile terminal, however, since the AGC has a wide operation range of 80 dB to 90 dB, the linearity, in dB conversion is lost to generate an error. As a result, an error occurs in the transmission power level in open-loop control. The gain varies depending on the temperature and frequency characteristics of the variable gain element constituting the AGC, and variations between the transmission and receive AGCs cause an error in power controllability.
Further, an AGC amplifier suitable for cost reduction uses a low-quality, low-linearity variable gain element. Considering reduction in power consumption, a low-power-consumption AGC amplifier also exhibits the same trend. For these reasons, demands have arisen for an AGC amplifier having high linearity.
Also, in integrating the AGC circuit, the circuit area must be minimized as much as possible. However, area reduction is limited in AGC circuits, for example, as disclosed in Japanese Examined Patent Publication No. 7-20034 and Japanese Unexamined Patent Publication No. 8-293748.
The present invention has been made to solve the conventional drawbacks, and has as its object to provide a digital automatic gain control linearizer which has high-precision linearity over a wide dynamic range and can be easily integrated, and a digital automatic gain control circuit using the same in auto gain control adopted in the radio field such as CDMA mobile terminals.
To achieve the above object, according to the main aspect of the present invention, there is provided a digital automatic gain control linearizer for controlling a variable gain control element for automatically controlling a gain, using an analog signal obtained by converting input digital data, comprising a memory for outputting upper and lower limit data respectively corresponding to upper and lower limits of a binary data range which can be expressed by a plurality of upper bits forming the digital data, and average calculation means for calculating an average of one and other data of the upper and lower limit data, and repeating average calculation a predetermined number of times using the calculated average as the other data, wherein average calculation by the average calculation means is repeated a number of times corresponding to a lower-bit value of the digital data except for the plurality of upper bits, and the variable gain control element is controlled in accordance with the average calculated by the average calculation means.
To achieve the above object, according to the secondary aspect of the present invention, there is provided a digital automatic gain control circuit comprising an automatic gain control loop including a variable gain control element having a reception gain controlled by the digital automatic gain control linearizer defined in the main aspect.
To achieve the above object, according to another secondary aspect of the present invention, there is provided a digital automatic gain control circuit comprising an automatic gain control loop including a variable gain control element having a transmission gain controlled by the digital automatic gain control linearizer defined in the main aspect.
An object of the present invention is to obtain linearity of an AGC amplifier in a mobile communication terminal having transmission and receive AGCs. The transmit AGC is connected to a power amplifier, whereas the receive AGC receives a signal proportional to the strength of the received signal. A received signal strength detection circuit (to be referred to as an RSSI) is connected to the receive AGC to generate digital RSSI information.
The RSSI signal is integrated by an integrator, and the integral result is used as a control signal for the digital AGC amplifier. If the linearity is maintained, the control signal level is proportional to the gain of the receive AGC amplifier for maintaining desired reception power in dB. The receive AGC linearizer is connected to the integrator, and generates a linear digital AGC control signal to compensate for nonlinearity of the variable gain element of the AGC amplifier.
The output of the receive AGC linearizer is connected to a digital-to-analog converter (to be referred to as a DAC). The DAC converts a receive AGC control signal into an analog control signal, which is input to the variable gain element of the receive AGC to control the gain of the AGC amplifier.
Similar to the receive AGC linearizer, the transmit AGC linearizer is connected to an RSSI integrator, and generates a linear transmit AGC control signal from an output from the integrator in order to compensate for nonlinearity of the transmission amplifier. The output of the transmit AGC linearizer is also connected to a DAC. This DAC for the transmit AGC converts a transmit AGC control signal into an analog control signal to control the gain of the variable gain element of the transmit AGC.
The digital AGC linearizer of the present invention is adopted in a mobile communication terminal having transmission and receive AGCs. The mobile communication terminal has a means for detecting the received digital signal.
The digital AGC linearizer of the receive AGC has a plurality of values representing reception characteristics, and a receive AGC control signal is generated from an output from the digital AGC linearizer. The reception digital AGC linearizer selects two of the values based on the detection output of the received signal, and determines an output from the digital AGC linearizer based on the detection output and selected two values.
The transmission digital AGC linearizer of the transmit AGC has a plurality of values representing transmission characteristics, and a transmit AGC control signal is generated from an output from the transmission digital AGC linearizer. The transmission digital AGC linearizer selects two of the values based on a terminal-side transmission power control signal included in a received signal, and determines an output from the transmission digital AGC linearizer based on the transmission power control signal and selected two values.
Outputs from the respective AGC linearizers are input to AD converters where the digital control signals are converted into analog control signals. The converted analog control signals control the gains of the receive and transmit AGCs.
Note that the ambient temperature may be detected to add this temperature condition as a parameter.
As is apparent from the above aspects, since the present invention employs a linearizer for calculating the middle point by sequential processing using only a simple adder and shift circuit, a digital automatic gain control circuit can be realized with a simple circuit arrangement without increasing the chip area upon integration and using any multiplier which increases current consumption.
The above and other advantages, features and additional objects of the present invention will become manifest to those versed in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.